A power source device is mounted on an electric vehicle that runs using an electric motor or on a hybrid vehicle that runs using both a combustion engine and an electric motor (see Patent Document 1). The power source device disclosed in Patent Document 1 includes: a battery assembly including a plurality of battery cells (secondary cells); a bus bar module attached to an upper portion of the battery assembly; a voltage detection line whose one side is connected to a voltage detection terminal for detecting the voltage of each battery cell; a smoke exhausting duct for exhausting gas generated from each battery cell when the pressure inside the battery cell rises; and a control board to which the other side of the voltage detection line is connected and which detects the voltage of each battery cell. The battery assembly is in an integrated state of being pinched by a pair of end plates and then bundled by a binding band or the like. In each battery cell, a positive electrode and a negative electrode protrude in the upper portion with an interval to each other. These battery cells are arranged in alternate orientations such that a positive electrode and a negative electrode are located adjacent to each other. Further, the bus bar module is formed by linking a plurality of bus bar storing portions for storing a plurality of bus bars for connecting a positive electrode and a negative electrode of adjacent battery cells so as to establish series connection of the battery cells.